High voltage bushing



United States Patent O HIGH VOLTAGE RUSHING Donald- L. Johnston andWilliam A.y Wooldridge, Pittsfield, Mass., asslgnors to General ElectricCompany, a corporation of New York Application April 18, 195s, serialNo. 501,845

9 Claims. (Cl. 174-31) This invention relates to bushings, and more inpartlcular to an improved high voltage bushing for electrical apparatusof the dielectric liuid filled type and a method for making the same.

High voltage bushings of the type used for handling potentials greaterthan about 69 kilovolts are generally comprised of a central axialconductor surrounded by a core of dielectric fluid impregnatedinsulating material immersed in a dielectric uid insulating medium. Atop and bottom ceramic shell separated by a metallic mounting flangesurround the core and central conductor, and provide means for retainingthe dielectric uid. A terminal is provided at the top end of thebushing, and the terminal may be connected to the central axialconductor, or to a cable passing through the conductor if the latter isin tubular form.

Since it is not readily possible to manufacture ceramic shells to closetolerances, the shells must be made suiciently large that a space existsbetween them and the insulating core they surround. This of courseresults in increased size of the bushing. The shells must be providedwith gaskets where they join the metal parts of the bushing, and cautionmust be taken during assembly that the various parts and gaskets areaccurately aligned. Although such bushings are satisfactory from thestandpoint of the function they serve, they are extremely costly tomanufacture.

In `order to reduce costs, bushings for use at voltages less than about69 kilovolts generally omit the bottom ceramic shell and dielectricfluid filled core and employ instead a solid insulating materialsurrounding the central conductor. The solid insulating materialgenerally extends for a considerable `distance upward through the topceramic shell. A dielectric fluid may surround the insulating materialin the upper portion of the bushing, but in this type of bushing thefull benefit is not derived from the insulating tiuid, and the solidinsulating material provides the majority of the insulation. Thus,although this type of bushing is much less costly than the type having abottom porcelain shell, the insulating characteristics are decreasedsince optimum use is not made of the dielectric fluid insulatingmaterial.

It is therefore an object of this invention to provide an improved highvoltage bushing for electrical apparatus.

It is also an object to provide an improved dielectric fluid illed highvoltage bushing without a bottom ceramic shell.

A further object of this invention is to eliminate the necessity forbottom ceramic shells in high voltage bushings -of the type employing adielectric lluid impregnated core surrounding a central conductor.

Still another object is to provide an improved method for fabricatinghigh voltage bushings of the type employing a dielectric fluid filledcore surrounding a central` conductor.

Briefly stated, in accordance with one embodiment of this invention, ahigh voltage bushing is provided com- 2,859,271 Patented Nov. 4, 1958prising a central axial conductor which may be solid or tubular. Asecondary tubular electrode electrically connected to the centralconductor radially surrounds the conductor and provides an annulardielectric fluid lled chamber surrounding the conductor. A dielectricfluid permeable core surrounds the secondary electrode, which is in turnsurrounded by a tube of solid insulating material to be more fullydescribed in the following paragraphs.l The top half `of the solidinsulating tube is surrounded by a ceramic shell, while the bottom halfof the insulating tube serves as an external surface of the bushing,Means are provided so that dielectric lluid may circulate by way of theannular chamber surrounding the central conductor to an annular chamberbetweenv the insulating tube and the ceramic shell and also to anannular chamber inside of the insulating tube below the bottom end ofthe core.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

In the drawing:

Fig. l is a partial cross-sectional view of a high voltage bushingillustrating one embodiment of this invention, and

Fig. 2 is an enlarged cross-sectional view of a portion of the highvoltage bushing of Fig. l.

Referring now to the drawing, a high voltage bushing is thereinillustrated having a conducting cable 10 extending from a top terminalassembly' 11 downwardly through a central conductor 12, and thenceemerging from the bottom end of the bushing. Although the centralconductor 12 is herein illustrated as being hollow, in some applicationsof the bushing, for example on circuit breakers, it may be desirablethat the cable be eliminated and the conductor be a solid memberextending from the top terminal through the bushing to a bottomterminal.

Por all except the top portion of its length the central conduct-or issurrounded by a unitary structure comprised of a secondary tubularelectrode 13, a dielectric uid permeable core 14, a solid electricalinsulating tube 15, and ground sleeve 16. The secondary electrode 13 hasan inside diameter larger than outside diameter of the central conductor12, so that an annular space 17 is provided between the electrode 13 andthe conductor. The electrode 13 is electrically connected and firmlyheld to the central conductor by means of a set screw 18 extendingthrough the thickened top end of the electrode and also by an end plate19 threaded to the bottom ends of the electrode and the conductor. Aplurality of holes 20 are drilled through the top and bottom ends of theelectrode for a purpose to be described later.

Surrounding the secondary electrode 13 is a dielectric uid impregnatedcore 14 `of the type conventionally ernployed in fluid filled bushings.This core may consist of a plurality of layers of paper wrapped aroundthe electrode 13. The core may have a plurality of electrostatic shieldmembers Wrapped integrally therewith according to the usual practice, aswell as an external shield 21.

Radially surrounding the core 14 is a tube 15 of solid insulatingmaterial. The insulating tube 15 extends from the top end of the coredownwardly to the end plate 19, thereby enclosing a chamber 22 below thebottom of the core, and is preferably formed by wrapping a plurality oflayers of resin impregnated laminar 'insulating material tightly aroundthe core 14 and then A ground sleeve 16 is shrunk iit over a machinedportion on the outside of the insulating tube with its bottom edgeagainst a machined shoulder 23 on the insulating tube.

The top outside surface of the bushing is comprised of a ceramicinsulator 25 and a metallic mounting flange 26. .The ceramic insulatorextends between the top terminal assembly and the mounting flange, andthe mounting iiange surrounds the central portion of the insulating tube15. In order to hold the ceramic insulator and mounting ange in place,the flange is provided with a shoulder 27, and tightening of theterminal assembly 1l against the ceramic insulator forces the flangeshoulder 27 against the top edge of the ground sleeve. If desired thediameter of the insulating tube may be reduced above the top edge of theground sleeve in order to provide a shoulder on the insulating tubeengaging with the shoulder 27 on the liange. The ceramic insulatorencloses a top chamber 2S surrounding the top end of the insulatingtube.

Therentire bushing is filled with a dielectric insulating fluid to alevel 29 above the top of the core, and the iiuid is free to circulatebetween the top chamber l28 and the bottom chamber 22 by way of holes 20and the annular space 17 surrounding the central conductor. Gaskets areprovided at the joints between the ceramic insulator, the top terminalassembly, and the mounting ange in order to prevent escape of thedielectric uid. A gasket may also be inserted between the top edge ofthe ground sleeve and the mounting flange.

The top terminal may be of any suitable conventional type, and will notbe further described herein other than to say that it should be providedwith a plug 30 to facilitate lling the bushing with dielectric uid andto measure the quantity of fluid in the bushing. It is also preferredthat a plug 33 be inserted in the bottom plate 19 to facilitate emptyingof the bushing.

According to usual practice in such bushings, an electrical connectionis made between the shield 21 and the mounting flange by means of aconductor 31, and electrical connection between the ground sleeve andthe mounting flange is assured by means of a screw 32 threaded in themounting flange and contacting the sleeve.

As has been previously stated, the lower end of the bushing is a unitarystructure comprised of the secondary tubular electrode 13, core 14,insulating tube 15, and ground sleeve 16. This assembly may be readilyfabricated by first wrapping the core tightly around the tubularelectrode, then wrapping layers of resin impregnated laminar materialtightly around the outside of the core. The structure is then heated toallow the resin impregnated material to form a solid insulatingmaterial. After cooling the insulating material is machined to thedesired dimensions, and the ground sleeve is shrunk lit over the centralportion thereof. This method produces a unitary structure that requiresa minimum of handling in the fabrication of the bushing. The assembly ofthe remaining portions of the bushing will be obvious to one skilled inthe art.,

From the above description it is readily seen that the bushing of thisinvention provides means for reducing the electrical stress between thebushing conductor and ground metal by providing an increase in thediameter of the insulating core surrounding the central conductorwithout increasing the bottom end diameter of the bushing as would berequired in the case of a bushing having a bottom ceramic shell.

The present invention is an improvement over the invention of thecopending application of E. F. Cronin and D. L. Johnston, Serial No.501,846, iiled April 18, 1955, and assigned to the present assignee.

It will be understood, of course, that while the form of the inventionherein shown and described constitutes a preferred embodiment of theinvention, it is not in tended herein to illustrate all of the possibleequivalent forms or ramifications thereof. It will also be understoodthat the words used are words of description rather than of limitation,and that various changes may be made without departing from the spiritor scope of the invention herein disclosed.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. In a high Voltage bushing for electrical apparatus, an axial centralconductor extending through said bushing, a secondary tubular electrodecoaxial with said conductor and separated therefrom providing an annularchamber surrounding said conductor, a dielectric uid impregnatedinsulating core surrounding at least a portion of said secondaryelectrode, solid insulating tubular means surrounding said core anddefining a lower dielectric fluid lled chamber, ceramic shell meanssurrounding Vthe upper portion of said tubular insulating means anddefining an upper dielectric fluid filled chamber, and means includingsaid annular chamber providing circulation of dielectric fluid betweensaid upper and lower chambers.

2. In a high voltage bushing for electrical apparatus, an axial centralconductor extending through said bushing, a secondary tubular electrodecoaxial with said conductor and electrically connected thereto, saidconductor and electrode deiining an annular dielectric iluid lledchamber surrounding said conductor, coaxial dielectric uid impregnatedinsulation core means wrapped around at least a portion of saidelectrode, coaxial solid insulating means surrounding said core meansand in contact therewith, ceramic shell means surrounding the upperportion of said bushing and deiining an Lipper dielectric fluid lilledchamber, said solid insulating means extending from the lower end ofsaid bushing into said upper chamber and defining a lower dielectricfluid lled chamber below the bottom of said core, and means includingsaid annular chamber providing circulation of dielectric fluid betweensaid upper chamber and said lower chamber.

3. In a high voltage bushing for electrical apparatus, an axialconductor extending through said Vbushing from a top terminal means, aceramic shell surrounding the top portion of said bushing and dening anupper chamber surrounding said central conductor, and aunitary'structure extending from the bottom of said bushing into saidupper chamber and surrounding said central conductor comprising acentral tubular electrode, a dielectric fluid iilled insulating corewrapped on said electrode, and a solid tubular insulation meanscoaxially surrounding said core and dening a lower chamber below thebottom of said core between said solid insulation means and said centralelectrode, said central electrode surrounding said central conductor andbeing electrically connected thereto and defining an annular chambersurrounding said central conductor, saidy annular chamber communicatingwith said upper and lower chambers.

4. A high voltage bushing for electrical apparatus comprising axialconducting means connected to a top terminal means and extending throughsaid bushing, upper external insulating means defining an upper chambersurrounding said conducting means, lower external insulating meansextending into said upper chamber, said lower external insulating meanscomprising a central dielectric uid permeable insulating means and anouter solid insulation means and defining a lower chamber below saidpermeable insulating means, and an annular chamber between saidconducting means and said lower external insulating means andcommunicating with said upper and lower chambers.

5. The bushing of claim 4 wherein said outer solid insulating means is aresin impregnated laminar material.`

6. A high voltage bushing for electrical apparatus com- I prising axialconducting means connected to top terminal means and extending throughsaid bushing, upper ceramic shell means adjacent said top terminal meansproviding an upper external surface for said bushing and defining anupper chamber surrounding said conducting means, and a lower externalinsulating means surrounding the lower portion of said conducting meansand extending into said upper chamber, secondary tubular electrode meanssurrounding said conducting means and electrically connected thereto,said lower insulating means comprising a dielectric uid permeableinsulating core means wrapped around said electrode means, and a solidinsulating means surrounding said core means and defining a lowerchamber surrounding said electrode means below said core means, Saidelectrode means being spaced from said conducting means to provide anannular chamber therebetween communicating with said upper and lowerchambers.

7. A high voltage bushing for electrical apparatus comprising axialconducting means connected to top terminal means and extending throughsaid bushing, upper ceramic shell means adjacent said top terminal meansproviding an upper external surface for said bushing and defining anupper chamber surrounding said conducting means, a lower externalinsulating means surrounding the lower portion of said conducting meansand extending into said upper chamber, and flanged mounting meanslpositioned at the lower end of said ceramic shell means and separatingsaid ceramic shell means from said lower external insulating means,secondary tubular electrode means surrounding said conducting means andelectrically connected thereto, said lower insulating means comprising adielectric fluid permeable insulating core means wrapped around saidelectrode means, and a solid insulating resin impregnated laminar meanssurrounding said core means and defining a lower chamber surroundingsaid electrode means -below said core means, said electrode means beingspaced from said conducting means to provide an annular chambertherebetween communicating with said upper and lower chambers.

8. A high voltage bushing for electrical apparatus comprising axialconducting means connected to top terminal means and extending throughsaid bushing, upper ceramic shell means having a top edge adjacent saidtop terminal means and a bottom edge adjacent a anged metallic mountingmeans, said ceramic shell means providing an upper external surface forsaid bushing and defining an upper chamber surrounding said `conductingmeans, and a lower external insulating means surrounding the lowerportion of said conducting means and extending through said flangedmounting means into said upper chamber,

secondary tubular electrode means surrounding said conducting means andbeing electrically connected thereto, said lower insulating meanscomprising a dielectric fluid permeable insulating core means spirallywrapped around said electrode means, a solid insulating resinimpregnated laminar means surrounding said core means and dening a lowerchamber surrounding said electrode means below said core means, and anannular ground sleeve means surrounding a portion of said solidinsulating means and being electrically connected to said anged mountingmeans, said electrode means being spaced from said conducting means toprovide an annular chamber surrounding said conducting means andcommunicating with said upper and lower chambers.

9. A lower external insulating member for a high voltage bushingcomprising a secondary axial tubular electrode means through which acentral conductor of said bushing extends, the inside diameter of saidelectrode being greater than the outside diameter of said centralconductor to provide an annular space surrounding said conductor, adielectric uid impregnated insulating core spirally wrapped `around saidelectrode means, a solid insulating resin impregnated laminar meanssurrounding said core means and extending below the lower end of saidcore means to provide an annular chamber surrounding the lower end ofsaid electrode means, and an annular ground sleeve means surrounding aportion of said solid insulating means, said electrode means extendingbeyond the ends of said core means and having holes drilled through thetop and bottom ends of said e1ectrode means beyond the extremities ofsaid core means.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES New Apparatus Bushing, Anderson and Johnson,

Trans. A. I. E. E., vol. 71, part III, 1952. Copy in 174-31.1.

